WASHINGTON  A new class of planets has been found orbiting stars besides our Sun, in a possible giant leap forward in the search for Earth-like planets that might harbor life, scientists said Tuesday.

"We can't quite see the Earth-like planets yet, but we are seeing their big brothers, and hopefully we will be bearing down on these small-mass planets soon," said Paul Butler of the Carnegie Institution of Washington, the co-discoverer of one of the new planets.

The two new planets are about 15 to 20 times more massive than Earth — approximately the mass of Neptune — and have diameters about two or three times the size of our home planet, astronomers said at a news conference at NASA headquarters.

That makes these new bodies different from most of the other so-called exoplanets found in the last decade outside our solar system. These other planets, more than 100 of them, are generally about the mass of Jupiter — about 318 times Earth's mass — and are thought to be balls of gas, completely inhospitable to life as Earthlings know it.

But the newly discovered planets indicate that planetary systems around other stars could have the same assortment of planets as in our solar system: big gassy ones like Jupiter, middle-weight rocky ones like Neptune , and just possibly, relatively small rocks like Earth.

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If scientists find an Earth-mass planet, they could then search for one located just the right distance from its star, making it temperate enough to allow for the presence of water on its surface, considered a requirement for life.

No one has ever seen an extrasolar planet. Most have been detected by looking for a characteristic wobble in a distant star, a sign that a planet's gravity is tugging on the star in a specific way.

Zipping Around the Stars

Butler and fellow planet-hunter Geoffrey Marcy of the University of California , Berkeley , discovered one of the Neptune-mass planets around a small star called Gliese 436, some 30 light-years away in the constellation Leo, a stone's throw in cosmic terms.

The other neptunian planet was discovered by Barbara McArthur of the University of Texas , Austin . This one is orbiting the star 55 Cancri in the constellation Cancer, about 40 light-years away. A light-year is about 6 trillion miles, the distance light travels in a year.

Both zip around their respective stars in three days or less, at a small fraction of the distance that Earth orbits the Sun.

The planet around 55 Cancri is the fourth planet detected there, but the others in that system fit the gas-giant mold, the scientists said.

The two new discoveries are the smallest planets found so far around Sun-like stars, the American astronomers said. They acknowledged that a European team of astronomers announced last week the discovery of a planet some 14 times Earth's mass, a so-called super Earth.

However, Alan Boss of the Carnegie Institution said credit for such discoveries is based on when academic papers are submitted for publication, not on when they are announced to the media. Under this rule, Boss said, the European team would get credit for discovering the third Neptune-mass planet.

"It's a stiff competition but we're quite friendly," Marcy said of the European planet-hunting team. "We recognize that the best science is done when there's a bit of tension, a bit of competition, but friendly, with science as the ultimate goal."